How To Test The Alternator (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma)

TEST 3: Checking The Alternator's F Circuit

For our third test, we'll make sure that the Field (F) circuit of the alternator's voltage regulator is getting 10 to 12 Volts.

This voltage is provided to the voltage regulator by the terminal labeled with the letter F in the illustration of the alternator connector above.

The 12 Volts that terminal F supplies to the alternator come from the IGN-GAU (20A) fuse located in the instrument panel fuse box (see illustration 2 of 2 above).

NOTE: The IGN-GAU Fuse (20A) is the fuse that supplies 12 Volts to the F and L terminals on all 1988-1993 2.8L Chevrolet S10 pickup, GMC S15 pickup and GMC Sonoma.

Let's get testing:

1. 1

   Reconnect the negative (-) battery cable to the battery's negative (-) post.

2. 2

   Disconnect the 2-wire pigtail connector from the alternator.

3. 3

   Connect the black multimeter test lead to the battery negative (-) terminal.
   
   IMPORTANT: The multimeter test lead must connect to a clean spot on the battery's negative (-) terminal

4. 4

   Connect the red multimeter test lead to the female terminal labeled with the letter F of the alternator's 2-wire pigtail connector (see the illustration above).

5. 5

   Have your helper turn the key to the ON position (RUN position) but ask him NOT to crank the engine.

6. 6

   Your multimeter should register 10 to 12 Volts DC.

Let's analyze your test result:

CASE 1: The multimeter registered 10 to 12 Volts DC. This is the correct and expected test result.

Your next and last step is to go to: TEST 4: Checking The Alternator's L Circuit.

CASE 2: The multimeter DID NOT register 10 to 12 Volts DC. Check the alternator fuse to see if its blown.

If the fuse is blown, replace it with a fuse of the same rating and repeat TEST 1.

If the alternator still does not charge the battery, go to: TEST 4: Checking The Alternator's L Circuit.

TEST 4: Checking The Alternator's L Circuit

For our fourth and final test, we'll check that terminal L of the alternator pigtail connector is delivering 10 to 12 Volts to the voltage regulator with the key in the ON position.

The Lamp (L) circuit does two important things:

• Illuminates the battery light on your instrument cluster when the alternator is not charging the battery.
• Activates (excites) the alternator's voltage regulator to begin charging the battery.

Let's get started:

1. 1

   Disconnect the alternator's 2-wire connector if it isn't already.

2. 2

   Place your multimeter in Volts DC mode.

3. 3

   Connect the black multimeter test lead to the negative (-) battery terminal.

4. 4

   Turn the key to the ON position but don't crank or start the engine.

5. 5

   With the red multimeter test lead, probe the female terminal labeled with the letter L of your alternator's connector.

6. 6

   Your multimeter should register 10 to 12 Volts DC.

Let's interpret your test result:

CASE 1: The multimeter registered 10 to 12 Volts DC. This is the correct and expected test result.

You can conclude that the alternator is bad if you have:

• Confirmed that battery voltage is 12.5 Volts DC and drops as the engine runs (TEST 1).
• Confirmed that the inline fusible link that protects the alternator's output wire is not blown (TEST 2).
• Confirmed that the IGN-GAU fuse is supplying 10 to 12 Volts to the F terminal (TEST 3).
• Confirmed in this last test section that the L terminal has 10 to 12 Volts DC.

CASE 2: The multimeter DID NOT register 10 to 12 Volts DC. This result tells you that one of three things have gone wrong:

• The fuse that supplies 12 Volts to the battery (charge) lamp is blown.
• An open-circuit problem has occurred in the L circuit wire between the connector and the instrument panel or fuse box.
• The battery lamp is blown (in the instrument cluster).

Although its beyond the scope of this tutorial to diagnose this issue, your next step is to find out why the L terminal does not have 10 to 12 Volts and resolve the problem.

More 2.8L Chevy S10 Pickup, GMC S15 Pickup, And GMC Sonoma Tutorials

You can find a complete list of tutorials for the 2.8L V6 Chevrolet S10 pickup, GMC S15 pickup, and GMC Sonoma in this index:

• GM 2.8L Index Of Articles.

Here's a sample of the articles, you'll find in the Index of Articles:

• How To Test The Fuel Pump (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
• How To Test Engine Compression (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
• How To Test The EGR Valve (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
• How To Test The Starter Motor (1988-1993 2.8L V6 Chevrolet S10 Pickup, GMC S15 Pickup).